Referring to FIG. 21, the following description discusses a structure of a conventional differential device 100. The differential device 100 is provided with a pinion shaft (drive pinion) 102 placed in its differential case 101, and this pinion shaft 102 has on one axial end thereof a pinion gear 106 that is engaged with a ring gear 108 of a differential transmission mechanism 107. The pinion shaft 102 is supported so as to freely rotate around its axis by tapered roller bearings 103 and 104 with single raceway that are placed in a manner so as to separate axially from each other. A companion flange 105, which is connected to a propeller shaft, not shown, is attached to the other axial end of the pinion shaft 102.
In the differential device 100, a bearing device, which rotatably supports the pinion shaft 102, is constituted by the tapered roller bearings 103 and 104 that rotatably support the pinion shaft 102 on its half way.
In the differential device 100 having such arrangement, since the rolling elements of the tapered roller bearings 103 and 104 are rollers, a greater frictional resistance is exerted on the tapered roller bearing 103 particularly on the pinion gear 106 side having a greater thrust load. For this reason, the rotary torque becomes greater, resulting in degradation in the efficiency of the differential device 100.